Each year in the United States 550,000 people develop upper-extremity movement deficits after stroke (hemiparesis). The recent success of mirror therapy (MT) is notable because it is a simple treatment for hemiparesis, and may be feasible for home use. MT uses a standard mirror to create a compelling illusion in which movements of the unimpaired limb appear as if they are being made by the impaired limb. We propose to complete a randomized placebo-controlled clinical trial of MT with a target enrollment of 100 chronic stroke patients. The therapy will consist of a standardized set of hand, wrist, and elbow movements completed in two daily 30-minute sessions, 5 times per week for 4 weeks. Patients assigned to the placebo treatment (identical therapy with an opaque divider rather than a mirror) will be crossed over to receive MT after a three month follow-up. The first goal of the study will be to determine whether a home-based form of MT is an effective treatment of hemiparesis. Providing evidence that mirror therapy could be administered at home would have enormous implications for the cost of treating hemiparesis, especially relative to alternative treatments that require a significant amount of therapist time (constraint-induced movement therapy) or expensive equipment (robotic training). Pilot data suggest that home-based mirror therapy could lead to clinically significant improvements in functioning. The second goal will be to determine the optimal dosing of MT by including weekly measures of improvement. The third goal will be to understand individual differences in the efficacy of mirror therapy. Previous MT studies have not reported, or lacked the power to test, why some patients benefit from MT and others do not, even though significant individual differences have been reported. Our use of a large sample of patients will allow us to assess predictors of therapeutic benefit. Being able to predict beneficial clinical outcomes would be quite innovative because this issue is rarely addressed in rehabilitation research more generally and is crucial to the real world task of treatment selection for individual patients.